Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

An apparatus and method for producing a luminescent device using a pulsed
electrical power feed. The pulsed feed produces a lower initial drop in
luminescent efficiency compared to a constant power feed. This method and
apparatus avoid traditional processes such as burn-in, used to establish
more uniform device performance.

Claims:

1. A method of operating an electronic device, comprising: providing a
first electrode; providing a second electrode; providing an organic
active material; connecting the organic active material to the first and
second electrodes to form a unit; and pulsing electrical power to the
unit.

2. The method of claim 1 wherein the pulsing rate is between 50 Hz and
1,000 Hz.

3. The method of claim 2 wherein the duty cycle is between 30% and 95%.

4. The method of claim 3 wherein the unit is a pixel.

5. The method of claim 3 wherein the unit is a sub-pixel.

6. An electronic device comprising: a first electrode; a second
electrode; an organic active material electrically connected to the first
and second electrodes to form a unit; and a source of pulsed electrical
power to the unit.

7. The electronic device of claim 6 wherein the electronic device is an
OLED display.

8. The electronic device of claim 6 wherein the electronic device is an
OLED lamp.

9. A method of making an OLED device comprising the steps of: providing a
first electrode; providing a second electrode; providing an organic
active material; connecting the organic active material to the first and
second electrodes to form a pixel; and providing a source of pulsed
electrical power to the pixel.

10. The method of claim 9 wherein the electrical power is pulsed at a
rate of between 50 Hz and 1,000 Hz.

11. The method of claim 10 wherein the duty cycle is between 30% and 95%.

12. The method of claim 11 wherein the pixel is a sub-pixel.

13. The method of claim 9 wherein the OLED device is an OLED display.

14. The method of claim 9 wherein the OLED device is an OLED lamp.

Description:

RELATED APPLICATION

[0001] This application claims priority under 35 U.S.C. §119(e) from
Provisional Application No. 61/233,600 filed Aug. 13, 2009 which is
incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

[0002] This disclosure relates in general to an electronic device. In
particular, it relates to a method and apparatus having a drive scheme to
minimize luminescent efficiency losses.

BACKGROUND INFORMATION

[0003] Increasingly, active organic molecules are used in electronic
devices. These active organic molecules have electronic or
electro-radiative properties including electroluminescence. Electronic
devices that incorporate organic active materials may be used to convert
electrical energy into radiation and may include a light-emitting diode,
light-emitting diode display, or diode laser.

[0004] One common characteristic of devices employing active organic
molecules is a significant loss of luminance in the first few hours of
operation, typically from 5 to 30% loss within the first 5 hours of
operation. While different materials show varying degrees of initial loss
of luminance, the electronic devices using these materials exhibit this
effect efforts are ongoing to address this problem. One solution is to
use a burn-in process to induce an initial luminance drop before the
electronic devices complete the manufacturing process. This "burn-in"
process can be achieved by operating the electronic device at high
temperature, or high current, for a designated time to induce the
required initial drop in luminance. At least two problems result from the
use of the burn-in process. One being the permanent lowering of device
efficiency, and the second being the additional process step required for
manufacturing, resulting in higher costs for a large volume manufacturing
process.

[0005] Alternatives are sought for avoiding the burn-in process to reduce
costs and mitigate the efficiency loss. Applications such as organic
light-emitting diode ("OLED") displays and general lighting are just
beginning to make inroads into consumer goods, and volume production will
be increasing every year for many years to come.

[0006] One method of manufacturing OLED devices involves forming discreet
pixel areas comprising several layers, including organic active material.
These pixels can be a single pixel, or composed of two or more
sub-pixels, for example, red, green and blue sub-pixels can be used to
form a single pixel in a display application. These pixels are typically
connected directly to a power bus to provide a voltage potential across
the pixel and resultant luminescence

[0007] There continues to be a need for improved devices for reducing
initial drop in luminance in display and lamp applications.

SUMMARY

[0008] In one embodiment the apparatus and method provide for a first and
second electrode, with one of the electrodes being an anode and one
electrode being a cathode. An organic active material, described in more
detail below, forms an electrical connection with the first and second
electrodes to form a unit. In one embodiment this unit is a pixel. Each
pixel can be formed from at least two sub-pixels, and in one embodiment
three sub-pixels form a pixel, with red, green and blue emissive
spectrums. Electrical power is delivered non-continuously, or pulsed, to
the unit. In one embodiment the pulsing can be distinct for each pixel,
sub-pixel or set of pixels. The pulsing rate can vary from 50 Hz up to
1,000 Hz, and the duty cycle, or percentage of time the power is "ON" is
30 to 95%. In one embodiment the pulsing rate and duty cycle can produce
many different scenarios, including alternating cycles of "ON-OFF", or
several cycles of "ON" followed by one or more cycles of "OFF", and
various other combinations to produce the stated pulsing rate and duty
time.

[0009] In one embodiment the apparatus and method can be an Organic Light
Emitting Diode (OLED) as a display for electronic devices such as cell
phones, PDA's, GPS's, music devices, desktop and laptop computers. In
another embodiment the OLED can be a lamp for general lighting purposes
in either indoor or outdoor applications.

[0010] In one embodiment, a substrate (such as glass) is useful as a base
for the electronic device. The term "organic electronic device" or
sometimes just "electronic device", is intended to mean a device
including one or more organic semiconductor layers or materials. An
organic electronic device includes, but is not limited to: (1) a device
that converts electrical energy into radiation (e.g., a light-emitting
diode, light emitting diode display, diode laser, or lighting panel), (2)
a device that detects a signal using an electronic process (e.g., a
photodetector, a photoconductive cell, a photoresistor, a photoswitch, a
phototransistor, a phototube, an infrared ("IR") detector, or a
biosensors), (3) a device that converts radiation into electrical energy
(e.g., a photovoltaic device or solar cell), (4) a device that includes
one or more electronic components that include one or more organic
semiconductor layers (e.g., a transistor or diode), or any combination of
devices in items (1) through (4).

BRIEF DESCRIPTION OF THE FIGURES

[0011] FIG. 1 is an illustration of an electronic device.

[0012] FIG. 2 is an illustration of one embodiment of waveforms used to
produce pulsed electrical power.

[0013] FIG. 3 is an illustration of one embodiment where pulsed power is
compared to continuous power application.

[0014]FIG. 4 is an illustration of one embodiment where improvement in
duty cycles vs. continuous power is provided for initial luminance drop
values.

DETAILED DESCRIPTION

[0015] One example of an electronic device comprising an organic
light-emitting diode ("OLED"), is shown in FIG. 1 and designated 100. The
device has an anode layer 110, a buffer layer 120, a photoactive layer
130, and a cathode layer 150. Adjacent to the cathode layer 150 is an
optional electron-injection/transport layer 140. Between the buffer layer
120 and the photoactive layer 130, is an optional
hole-injection/transport layer (not shown).

[0016] As used herein, the term "buffer layer" or "buffer material" is
intended to mean electrically conductive or semiconductive materials and
may have one or more functions in an organic electronic device, including
but not limited to, planarization of the underlying layer, charge
transport and/or charge injection properties, scavenging of impurities
such as oxygen or metal ions, and other aspects to facilitate or to
improve the performance of the organic electronic device. Buffer
materials may be polymers, oligomers, or small molecules, and may be in
the form of solutions, dispersions, suspensions, emulsions, colloidal
mixtures, or other compositions. The term "hole transport" when referring
to a layer, material, member, or structure, is intended to mean such
layer, material, member, or structure facilitates migration of positive
charges through the thickness of such layer, material, member, or
structure with relative efficiency and small loss of charge. The term
"electron transport" when referring to a layer, material, member or
structure, is intended to mean such a layer, material, member or
structure that promotes or facilitates migration of negative charges
through such a layer, material, member or structure into another layer,
material, member or structure. The term "hole injection" when referring
to a layer, material, member, or structure, is intended to mean such
layer, material, member, or structure facilitates injection and migration
of positive charges through the thickness of such layer, material,
member, or structure with relative efficiency and small loss of charge.
The term "electron injection" when referring to a layer, material,
member, or structure, is intended to mean such layer, material, member,
or structure facilitates injection and migration of negative charges
through the thickness of such layer, material, member, or structure with
relative efficiency and small loss of charge.

[0017] The device may include a support or substrate (not shown) that can
be adjacent to the anode layer 110 or the cathode layer 150. Most
frequently, the support is adjacent the anode layer 110. The support can
be flexible or rigid, organic or inorganic. Generally, glass or flexible
organic films are used as a support. The anode layer 110 is an electrode
that is more efficient for injecting holes compared to the cathode layer
150. The anode can include materials containing a metal, mixed metal,
alloy, metal oxide or mixed oxide. Suitable materials include the mixed
oxides of the Group 2 elements (i.e., Be, Mg, Ca, Sr, Ba, Ra), the Group
11 elements, the elements in Groups 4, 5, and 6, and the Group 8-10
transition elements. If the anode layer 110 is to be light transmitting,
mixed oxides of Groups 12, 13 and 14 elements, such as indium-tin-oxide,
may be used. As used herein, the phrase "mixed oxide" refers to oxides
having two or more different cations selected from the Group 2 elements
or the Groups 12, 13, or 14 elements. Some non-limiting, specific
examples of materials for anode layer 110 include, but are not limited
to, indium-tin-oxide ("ITO"), aluminum-tin-oxide, gold, silver, copper,
and nickel. The anode may also comprise an organic material such as
polyaniline, polythiophene, or polypyrrole. The IUPAC number system is
used throughout, where the groups from the Periodic Table are numbered
from left to right as 1-18 (CRC Handbook of Chemistry and Physics,
81st Edition, 2000).

[0019] The photoactive layer 130 may typically be any organic
electroluminescent ("EL") material, including, but not limited to, small
molecule organic fluorescent compounds, fluorescent and phosphorescent
metal complexes, conjugated polymers, and mixtures thereof. Examples of
fluorescent compounds include, but are not limited to, pyrene, perylene,
rubrene, coumarin, derivatives thereof, and mixtures thereof. Examples of
metal complexes include, but are not limited to, metal chelated oxinoid
compounds, such as tris(8-hydroxyquinolato)aluminum (Alq3);
cyclometalated iridium and platinum electroluminescent compounds, such as
complexes of iridium with phenylpyridine, phenylquinoline, or
phenylpyrimidine ligands as disclosed in Petrov et al., U.S. Pat. No.
6,670,645 and Published PCT Applications WO 03/063555 and WO 2004/016710,
and organometallic complexes described in, for example, Published PCT
Applications WO 03/008424, WO 03/091688, and WO 03/040257, and mixtures
thereof. Electroluminescent emissive layers comprising a charge carrying
host material and a metal complex have been described by Thompson et al.,
in U.S. Pat. No. 6,303,238, and by Burrows and Thompson in published PCT
applications WO 00/70655 and WO 01/41512. Examples of conjugated polymers
include, but are not limited to poly(phenylenevinylenes), polyfluorenes,
poly(spirobifluorenes), polythiophenes, poly(p-phenylenes), copolymers
thereof, and mixtures thereof.

[0020] The particular material chosen may depend on the specific
application, potentials used during operation, or other factors. The EL
layer 130 containing the electroluminescent organic material can be
applied using any number of techniques including vapor deposition,
solution processing techniques or thermal transfer. In another
embodiment, an EL polymer precursor can be applied and then converted to
the polymer, typically by heat or other source of external energy (e.g.,
visible light or UV radiation).

[0021] Optional layer 140 can function both to facilitate electron
injection/transport, and can also serve as a confinement layer to prevent
quenching reactions at layer interfaces. More specifically, layer 140 may
promote electron mobility and reduce the likelihood of a quenching
reaction if layers 130 and 150 would otherwise be in direct contact.
Examples of materials for optional layer 140 include, but are not limited
to, metal chelated oxinoid compounds, such as
tris(8-hydroxyquinolato)aluminum (Alq3),
bis(2-methyl-8-quinolinolato)(para-phenyl-phenolato)aluminum(III) (BAIQ),
and tetrakis-(8-hydroxyquinolinato)zirconium (IV) (ZrQ); and azole
compounds such as 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole
(PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole
(TAZ), and 1,3,5-tri(phenyl-2-benzimidazole)benzene (TPBI); quinoxaline
derivatives such as 2,3-bis(4-fluorophenyl)quinoxaline; phenanthrolines
such as 4,7-diphenyl-1,10-phenanthroline (DPA) and
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (DDPA); and mixtures
thereof. Alternatively, optional layer 140 may be inorganic and comprise
BaO, LiF, Li2O, or the like.

[0022] The cathode layer 150 is an electrode that is particularly
efficient for injecting electrons or negative charge carriers. The
cathode layer 150 can be any metal or nonmetal having a lower work
function than the first electrical contact layer (in this case, the anode
layer 110). As used herein, the term "lower work function" is intended to
mean a material having a work function no greater than about 4.4 eV. As
used herein, "higher work function" is intended to mean a material having
a work function of at least approximately 4.4 eV.

[0023] Materials for the cathode layer can be selected from alkali metals
of Group 1 (e.g., Li, Na, K, Rb, Cs,), the Group 2 metals (e.g., Mg, Ca,
Ba, or the like), the Group 12 metals, the lanthanides (e.g., Ce, Sm, Eu,
or the like), and the actinides (e.g., Th, U, or the like). Materials
such as aluminum, indium, yttrium, and combinations thereof, may also be
used. Specific non-limiting examples of materials for the cathode layer
150 include, but are not limited to, barium, lithium, cerium, cesium,
europium, rubidium, yttrium, magnesium, samarium, and alloys and
combinations thereof.

[0024] In other embodiments, additional layer(s) may be present within
organic electronic devices. For example, a layer (not shown) between the
buffer layer 120 and the EL layer 130 may facilitate positive charge
transport, band-gap matching of the layers, function as a protective
layer, or the like. Similarly, additional layers (not shown) between the
EL layer 130 and the cathode layer 150 may facilitate negative charge
transport, band-gap matching between the layers, function as a protective
layer, or the like. Layers that are known in the art can be used. In
addition, any of the above-described layers can be made of two or more
layers. Alternatively, some or all of inorganic anode layer 110, the
buffer layer 120, the EL layer 130, and cathode layer 150, may be surface
treated to increase charge carrier transport efficiency. The choice of
materials for each of the component layers may be determined by balancing
the goals of providing a device with high device efficiency with the cost
of manufacturing, manufacturing complexities, or potentially other
factors.

[0025] The different layers may have any suitable thickness. In one
embodiment, inorganic anode layer 110 is usually no greater than
approximately 500 nm, for example, approximately 10-200 nm; buffer layer
120, is usually no greater than approximately 250 nm, for example,
approximately 50-200 nm; EL layer 130, is usually no greater than
approximately 100 nm, for example, approximately 50-80 nm; optional layer
140 is usually no greater than approximately 100 nm, for example,
approximately 20-80 nm; and cathode layer 150 is usually no greater than
approximately 100 nm, for example, approximately 1-50 nm. If the anode
layer 110 or the cathode layer 150 needs to transmit at least some light,
the thickness of such layer may not exceed approximately 100 nm. In
organic light emitting diodes (OLEDs), electrons and holes, injected from
the cathode 150 and anode 110 layers, respectively, into the EL layer
130, form negative and positively charged polar ions in the polymer.
These polar ions migrate under the influence of the applied electric
field, forming a polar ion exciton with an oppositely charged species and
subsequently undergoing radiative recombination. A sufficient potential
difference between the anode and cathode, usually less than approximately
12 volts, and in many instances no greater than approximately 5 volts,
may be applied to the device. The actual potential difference may depend
on the use of the device in a larger electronic component. In many
embodiments, the anode layer 110 is biased to a positive voltage and the
cathode layer 150 is at substantially ground potential or zero volts
during the operation of the electronic device. A battery or other power
source(s) may be electrically connected to the electronic device as part
of a circuit but is not illustrated in FIG. 1.

[0026] FIG. 2 illustrates two embodiments of waveforms used to provide
pulsed electrical power. In one embodiment the OFF period can be
characterized as zero voltage. In another embodiment the OFF period can
be characterized by a negative voltage, such as -5 volts. Typical OFF
voltages can be from zero to -8 volts. The supplied current can be any
value to provide desired luminescent intensity, in the embodiments shown
the current is 160 mA/cm2. Typical frequencies range from 50-1000 Hz
with duty cycles ranging from 30-95%.

[0027] FIG. 3 illustrates one example of differences in initial luminance
drop associated with a direct, also called continuous, power supply and
the pulsed system. A single substrate is used to minimize variation
between pixels, while direct current (DC) is supplied to one pixel, while
a pulsed current at 100 Hz and 95% duty cycle is supplied to a second
pixel. Both pixels receive 160 mA/cm2 while in the ON state. The
differences in the first 20 hours of operation, indicated by the circled
portion of FIG. 3, demonstrates a smaller initial drop in luminance for
the pulsed arrangement, and maintenance of a higher luminance for
subsequent time of operation. The time axis for the pulsed system is
adjusted, to equate the ON time for the direct and pulsed systems.

[0028]FIG. 4 illustrates several repetitions of the comparison discussed
in FIG. 3, for performance measurements using several pixels on one
substrate. T97 and T70 indicate the difference in pixel
luminance for 97% of initial luminance and 70% of initial luminance,
respectively. The magnitude of the initial drop is largest during the
first stage of operation, and differences between direct and pulsed
operation are also largest at this stage, as indicated by the T97
results. The pulsed drive data indicates lower initial luminance drop
values than that of continuous power application, with 2 to 10 times
performance improvement. In addition, no burn-in is required for high
volume manufacturing, saving both time and money using a pulsed drive
scheme.

[0029] For a radiation-emitting organic active layer, suitable
radiation-emitting materials include one or more small molecule
materials, one or more polymeric materials, or a combination thereof. A
small molecule material may include any one or more of those described
in, for example, U.S. Pat. No. 4,356,429 ("Tang"); U.S. Pat. No.
4,539,507 ("Van Slyke"); U.S. Patent Application Publication No. US
2002/0121638 ("Grushin"); or U.S. Pat. No. 6,459,199 ("Kido").
Alternatively, a polymeric material may include any one or more of those
described in U.S. Pat. No. 5,247,190 ("Friend"); U.S. Pat. No. 5,408,109
("Heeger"); or U.S. Pat. No. 5,317,169 ("Nakano"). An exemplary material
is a semiconducting conjugated polymer. An example of such a polymer
includes poly(paraphenylenevinylene) (PPV), a PPV copolymer, a
polyfluorene, a polyphenylene, a polyacetylene, a polyalkylthiophene,
poly(n-vinylcarbazole) (PVK), or the like. In one specific embodiment, a
radiation-emitting active layer without any guest material may emit blue
light.

[0030] For a radiation-responsive organic active layer, a suitable
radiation-responsive material may include a conjugated polymer or an
electroluminescent material. Such a material includes, for example, a
conjugated polymer or an electro- and photo-luminescent material. A
specific example includes
poly(2-methoxy,5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene) ("MEH-PPV")
or a MEH-PPV composite with CN-PPV.

[0031] For a hole-injecting layer, hole-transport layer, electron-blocking
layer, or any combination thereof, a suitable material includes
polyaniline ("PANI"), poly(3,4-ethylenedioxythiophene) ("PEDOT"),
polypyrrole, an organic charge transfer compound, such as
tetrathiafulvalene tetracyanoquinodimethane ("TTF-TCQN"), a
hole-transport material as described in Kido, or any combination thereof.

[0033] For an electronic component, such as a resistor, transistor,
capacitor, etc., the organic layer may include one or more of thiophenes
(e.g., polythiophene, poly(alkylthiophene), alkylthiophene,
bis(dithienthiophene), alkylanthradithiophene, etc.), polyacetylene,
pentacene, phthalocyanine, or any combination thereof.

[0034] Examples of an organic dye include
4-dicyanmethylene-2-methyl-6-(p-dimethyaminostyryl)-4H-pyran (DCM),
coumarin, pyrene, perylene, rubrene, a derivative thereof, or any
combination thereof.

[0035] Examples of an organometallic material include a functionalized
polymer comprising one or more functional groups coordinated to at least
one metal. An exemplary functional group contemplated for use includes a
carboxylic acid, a carboxylic acid salt, a sulfonic acid group, a
sulfonic acid salt, a group having an OH moiety, an amine, an imine, a
diimine, an N-oxide, a phosphine, a phosphine oxide, a β-dicarbonyl
group, or any combination thereof. An exemplary metal contemplated for
use includes a lanthanide metal (e.g., Eu, Tb), a Group 7 metal (e.g.,
Re), a Group 8 metal (e.g., Ru, Os), a Group 9 metal (e.g., Rh, Ir), a
Group 10 metal (e.g., Pd, Pt), a Group 11 metal (e.g., Au), a Group 12
metal (e.g., Zn), a Group 13 metal (e.g., Al), or any combination
thereof. Such an organometallic material includes a metal chelated
oxinoid compound, such as tris(8-hydroxyquinolato)aluminum (Alq3); a
cyclometalated iridium or platinum electroluminescent compound, such as a
complex of iridium with phenylpyridine, phenylquinoline, or
phenylpyrimidine ligands as disclosed in published PCT Application WO
02/02714, an organometallic complex described in, for example, published
applications US 2001/0019782, EP 1191612, WO 02/15645, WO 02/31896, and
EP 1191614; or any mixture thereof.

[0036] Examples of a conjugated polymer include a poly(phenylenevinylene),
a polyfluorene, a poly(spirobifluorene), a copolymer thereof, or any
combination thereof.

[0037] Selecting a liquid medium can also be an important factor for
achieving one or more proper characteristics of the liquid composition. A
factor to be considered when choosing a liquid medium includes, for
example, viscosity of the resulting solution, emulsion, suspension, or
dispersion, molecular weight of a polymeric material, solids loading,
type of liquid medium, boiling point of the liquid medium, temperature of
an underlying substrate, thickness of an organic layer that receives a
guest material, or any combination thereof.

[0038] In some embodiments, the liquid medium includes at least one
solvent. An exemplary organic solvent includes a halogenated solvent, a
hydrocarbon solvent, an aromatic hydrocarbon solvent, an ether solvent, a
cyclic ether solvent, an alcohol solvent, a glycol solvent, a glycol
ether solvent, an ester or diester solvent, a glycol ether ester solvent,
a ketone solvent, a nitrile solvent, a sulfoxide solvent, an amide
solvent, or any combination thereof.

[0054] In another embodiment, the liquid medium includes water. A
conductive polymer complexed with a water-insoluble colloid-forming
polymeric acid can be deposited over a substrate and used as a
charge-transport layer.

[0055] Many different classes of liquid medium (e.g., halogenated
solvents, hydrocarbon solvents, aromatic hydrocarbon solvents, water,
etc.) are described above. Mixtures of more than one of the liquid medium
from different classes may also be used.

[0056] The liquid composition may also include an inert material, such as
a binder material, a filler material, or a combination thereof. With
respect to the liquid composition, an inert material does not
significantly affect the electronic, radiation emitting, or radiation
responding properties of a layer that is formed by or receives at least a
portion of the liquid composition.

[0057] It is to be appreciated that certain features of the invention
which are for clarity, described above in the context of separate
embodiments, may also be provided in combination in a single embodiment.
Conversely, various features of the invention that are, for brevity
described in the context of a single embodiment, may also be provided
separately or in any subcombination. Further, reference to values stated
in ranges includes each and every value within that range.